The present invention relates to devices for the real time display by holographic interferometry of deformations of vibrating or deformable structures.
In numerous fields, it is useful to be able to carry out a non-destructive control or inspection on stressed mechanical parts. In the case of slow deformations, it is known to use devices based on conventional holography, observation being made possible by the memory effect inherent in the holographic media. The recording of two successive wave fronts with the same reference beam and the restoration by this reference beam of the recorded fringes makes it possible to create an interference pattern corresponding to the deformations. The localization of the fringes is directly linked with the deformations occurring between the recorded states. High resolution photographic supports are used as the recording medium, but they require chemical developing and a very precise repositioning, and, particularly in the case of vibrating structures, it is not possible to follow the deformations of the structures.
To provide a solution to this problem, it is known to use as the recording medium photosensitive electrooptical materials such as bismuth and silicon oxides (BSO) or bismuth and germanium oxide (BGO). In the case of such materials, the network of fringes created by the interference of the wave front coming from the object and the reference wave front induces in real time, to within the recording time of the material, a system of index layers constituting a hologram characterizing the object wave front. It diffracts part of the energy of the incident reference wave front in accordance with a diffracted wave, whose wave front is isomorphic of the object wave front. On replacing a mirror perpendicular to the path of the reference wave transmitted by the crystal in order to reflect said reference wave onto the crystal, the system of index layers inscribed in the crystal gives rise to a conjugate wave of the object wave on the basis of said reflected reference beam.
Such a property is used in the construction of a device for the display of deformable structure deformations by holographic interferometry. This device forms the subject matter of French patent application No. 7,708,627 filed on Mar. 23, 1977.
This device is used either for revealing the slow deformation of an object between two exposures or for studying the rapid vibrations of an object or part of an object. In the latter case, if the recording material exposure recording time is long compared with the vibration cycle and if the maximum amplitude .DELTA.e of the deformation does not exceed a few dozen times the radiation wavelength .lambda., the image obtained then corresponds to the image of the object to be analysed modulated by black fringes corresponding to the zeros of the Bessel function ##EQU1##
This device functions by retroreflection of the reference wave, generation of the conjugate wave front in the recording material and separation of this conjugate wave front from the incident object wave front by means of a semi-transparent plate. The reference wave consequently passes into the crystal twice and bearing in mind the absorption coefficients at the wavelengths used of about 500 nm, an optimum material thickness of about 2 mm is obtained.
However, such a device does not make it possible to observe interferograms of vibrating objects larger than 6 or 7 cm. The essential limitation is the energy available in the image plane generated by the conjugate wave front. This is in fact the energy of the retroreflected reference beam fraction diffracted by the photo-induced index system and the higher the modulation factor m of the fringes the higher the diffraction efficiency and it is in fact proportional to the square of m.
However, the modulation factor of the fringes is dependent on the ratio of the interfering beams. In the envisaged application, this ratio is approximately 10.sup.-3 to 10.sup.-4, corresponding to an extremely low modulation factor of the fringes of approximately 2.times.10.sup.-2. Compared with a modulation factor of 1, the energy available in the image is much lower by a factor of 4.times.10.sup.-4. Another consequence of the low modulation factor is that the recording time is longer. To obtain interferograms of diffusing objects, it is also necessary to have optimum conditions with regards to the quality of the crystal, the planarity of the reference wave, the crystal faces, etc.
Attempts have been made to obviate these disadvantages for obtaining interferograms of large 3D objects without it being necessary to perfect the operating conditions, by using wave coupling phenomena in thick photosensitive supports, such as the BSO monocrystal.
Physically, wave coupling leads to a significant energy transfer from the reference beam to the object beam after passing through the crystal. The directly transmitted object beam intensity is in practice increased by a factor of 2 to 3 in the presence of a reference beam which, under these conditions, acts as a pumping wave.
Thus, under these conditions, it is possible to discern the interferogram of the object on the path of the object beam emerging from the recording medium without being excessively disturbed by the radiation directly transmitted by the object. This leads to a significant simplification to the device by eliminating the shutters or mirrors which are indispensible in the prior art device operating by generation of the conjugate wave front.